Few monomers are commercially produced in the world on so large a .scale as vinyl chloride monomer ("VCM"). The U.S. production alone of VCM was about 7.7 billion pounds in the year 1984, most of which was used in the production of poly(vinyl chloride), the remainder was used for the production of copolymers of VCM with vinylidene chloride, graft copolymers of vinyl chloride on methylmethacrylate, polybutadiene, ethylene-propylene elastomer, etc.
Despite this scale of production, not much attention has been accorded the exacting requirements for the commodity VCM, except of course by those charged with the responsibility of producing on-spec product VCM. Product VCM, referred to as "finished" VCM, is limited to the following: HCl 0.5 parts per million (ppm) by weight; acetylene (C.sub.2 H.sub.2) 0.2 ppm; caustic (NaOH) 0.3 ppm; and, water 100 ppm. Many drugs for human consumption have less stringent purity specifications.
As of the present time, VCM produced in a conventional commercial VCM plant is derived from vinyl chloride VCl in the overhead of a VCl distillation column which overhead typically contains from about 50-500 ppm HCl and 10-300 ppm water. This VCl overhead, after it is condensed, is stripped in a stripping column to reduce the level of HCl which is taken overhead. The bottoms from the stripper, still containing in the range from about 1-50 ppm HCl, is scrubbed either by contact with caustic solution, or by upflow percolation through a bed of solid caustic.
The term "vinyl chloride monomer" (VCM) is used when the VCl has been purified, that is "finished", so that it meets product VCM specifications.
Those operating a VCM plant utilizing such a process, recognize that corrosion is the overriding problem in the stripping column. This problem is exacerbated because the corrosion process contributes ferric salts to the VCM. This is an impurity, among others, which can victimize an otherwise meticulously operated polymerization process because it produces off-spec poly(vinyl chloride).
The maintenance of solid caustic scrubbing beds is an unenviable task due to VCl emissions when the beds are opened to be recharged with fresh caustic pellets. Moreover, because of their very nature, they are subject to channeling and variability in contact efficiency even when channeling is minimal. The result is that the purity of the VCM is unreliable. To overcome the problem, the beds are greatly overdesigned. A concomitant of overdesigning the beds is the added risk of generating acetylene due to dehydrochlorination of VCl in "hot spots" in a caustic bed.
This problem of acetylene formation was recognized and addressed in a purification process for VCM disclosed in Japanese Pat. No. 57/2009234 A2 (12/22/82) which process removed HCl without forming acetylene byproduct by passing the liquid VCl through a column packed with a porous powder of an alkaline earth metal hydroxide or oxide, optionally supported on silica or alumina, with an avg. particle diam. of 3.5 mm and surface area 4.5 m.sup.2 /g.
Some commercial plants utilize caustic wash scrubbers in which the VCl from either the overhead of the vinyl chloride column, or the bottoms of the vinyl chloride stripper, is scrubbed by contact with an aqueous solution of NaOH at ambient temperature. But there was no reason for these plants to operate at sub-25.degree. F. temperatures, since it was known that the solubility of water in VCM at temperatures as low as -12.degree. C. (10.degree. F.) was 230 ppm (see "Solubility of Water in Vinyl Chloride" by Clarke, C. E. et al Can. J. Chem., 59(5), 768-71 1981). As will readily be appreciated, there was no logical reason to expect that a solubility of 230 ppm of water in VCM at 10.degree. F. would lead to the use of a cold caustic solution to dry VCl so that it contains less than 100 ppm water, and meet the `water spec` for VCM.
As will presently be apparent from the data presented hereinafter, it was discovered that the solubility of water in VCM drops off when there is caustic solution present, and the caustic solution is at a temperature below about 25.degree. F.(-3.89.degree. C.). It is this unexpected phenomenon which is the basis for the process of this invention.
As one would expect, the low temperature does not adversely affect the ability of the NaOH to neutralize the HCl present, but it unexpectedly suppresses the formation of by-product acetylene. Coincidentally, though the difference in specific gravities of the VCM and caustic solution diminishes with decreasing temperature, it is sufficiently large at a temperature below -4.degree. C. to permit a separation of the VCM and aqueous phases which separation is effected in a settling tank.
It was never realized that the low solubility of water in cold VCM was the key to using an aqueous stream to dry VCl. Washing wet VCl with cold caustic made it possible to meet the stringent specifications of product VCM. It is this singular effect of temperature on the solubility of water in VCM which permits the effective and reproducibly reliable results obtained with the process of this invention.
Further, the solubility of Na in VCM and that of water in VCM are such that it is possible to meet the stringent specifications of product VCM. It is these physical parameters of the components which permit the effective and reproducibly reliable results obtained with the process of this invention.